1. Field of the Invention
The present invention relates generally to a method for measurement of distance by laser light. The present invention particularly concerns distance measurement by laser light utilizing interference of the laser light.
2. Description of the Prior Art
Trials for accurate measurement of distance to an object by using laser light interference having good directivity has been recently proposed. There are reports of measurements of 100 nm, as an example of measurement of short distance, and 4.times.10.sup.5 km as an example of measurement of long distance to surface of the moon.
Principles of distance measurement by laser light are as follows:
(i) Projecting a laser light beam to an object, and forming interference of laser light between reflected lights from (1) a reference reflector disposed at a known position and (2) from the object, to make interference fringe patterns, from which distance is computed.
This method can produce very accurate measurement because accuracy of the measurement is beyond 1/10 of wavelength .lambda. of the laser light. However, this method has a shortcoming in that accurate measurement can be made only when the number of interference fringes can be accurately given.
As an improvement of the above-mentioned method, a method shown in FIG. 1 has been proposed wherein a laser light beam is frequency modulated by an acousto-optical modulator to emit a frequency-modulated light beam in a direction making a diffraction angle against a non-diffracted straight path component, and is projected to and reflected from an object. The reflected light is superposed with a reference light which is the frequency modulated light, and then the phase of beat signal obtained by superposing the laser lights is examined. Such an improved conventional method is elucidated more in detail with reference to FIG. 1.
As shown in FIG. 1, a laser light emitted from a laser oscillator 1 and having angular frequency .omega. is given to a modulator 4, which is for instance, an acousto-optical modulator. The modulator 4 is driven by an ultrasonic signal having an angular frequency .omega..sub.0 oscillated by a local oscillator 2 and amplified by a driver 3. By passing through the modulator 4, the laser light is divided into two components, namely, a modulated component having an angular frequency of .omega.+.omega..sub.0, and a non-diffracted component having the angular frequency of .omega.. The modulated light is given to a detector 6 which is disposed close to the laser oscillator 1. On the other hand, non-diffracted component is projected to an object 5 and reflected light from the object 5 is received also by the detector 6. In this system, the laser light reflected by the distance measurement object 5 has a phase difference .PHI.=2.pi.N+.phi., wherein N is a positive integer, relative to the phase of the non-modulated component part cos .omega.t of the output from the modulator 4. Thus, a beat signal cos (.omega..sub.0 t+.phi.) is issued from the detector 6 and is passed through an IF amplifier 7 to a phase detector 8, which issues signals for the phase difference .PHI. in forms of sin .phi. and cos .phi.. The output signals cos .phi. and sin .phi. are given to amplifiers 9 and 9', respectively, to issue output signals cos .phi. and sin .phi..
This conventional method gives measured distance L from the laser oscillator 1 to the object 5 in the following equation (1): ##EQU1## wherein, C is light velocity.
The first term in the parenthesis of the equation (1) shows that the measured distance L by this method includes uncertainty due to multiple solutions corresponding to respective ones of the integer N. The second term in the parenthesis of the equation (1) shows the accuracy of this measurement, and in case the measurement is made with an accuracy of 2.pi./100 radian, by using a laser light of 1 .mu.m wavelength, a very high accuracy of 0.01 .mu.m (=10 nm) is achievable.
In this method, in order to obtain the absolute value of the measured distance, a determination of the integer N becomes necessary. For instance, there is a disclosure in Optical Engineering, Vol. 20, 1981, P. 129, such that beat signals are produced in a classified order from five lines, i.e., R lines and P lines of a CO.sub.2 laser, to finally produce an interference beat wave of a wavelength of 57.4 m. Though having a high accuracy, this method has a shortcoming such that it is usable only for a distance measuring of within 1.5 m.
(ii) Another conventional method does not use the modulation element 4. This other method is such that the time of a round trip of the light from the laser beam oscillator 1 to the object 5 is measured and the distance is computed. In this method, the laser light is usually pulse light. The accuracy is determined by time resolution .tau. of the detector 6 and pulse width T of the laser light. In a special example, wherein .tau. is .tau..perspectiveto.20 ps and T.perspectiveto.200 ps, it was possible to achieve a measurement accuracy of 3 cm. The problem of this method is the difficulty of obtaining very short time width T of the pulse. For instance, in order to obtain the pulse width T of pico second order, it is necessary to utilize Q-switching, wherein achievement of high speed repetition is difficult. In the distance measurement of a moving object or in a process of producing an image signal by processing distance measuring signals, the repetition frequency of the pulse must be considerably high, and in such case the pulse width T must be selected fairly long. Since practically usable minimum measured time is 1 .mu.s or the like, wherein the laser light travels about 300 m, accuracy or minimum measurable distance becomes 150 m.
That is, distance measurement by using the conventional methods is limited to be usable only for the ranges of 1.5 m or shorter or that of 150 m or longer, and it has been impossible to accurately measure the distance between 10 m and 150 m by laser light.